This invention relates to a magnetic circuit structure for a pick up cartridge of moving magnet type.
Hitherto, as a pick up cartridge of this type (this is referred to as a mere "cartridge," hereinafter), those shown in FIGS. 1 and 2 are known. A cartridge shown in FIG. 1 is constructed as follows. A flat armature 3 magnetized in the direction of its thickness is secured to one end of a cantilever 2 on the other end of which a phonograph needle 1 is mounted in such a manner that the armature is perpendicular to the axial direction of the cantilever 2, and this flat armature 3 is supported tiltably and oscillatably about a pivot 5 within a yoke 4. In inclination and oscillation of the flat armature (A condition that the armature is tilted and biased by oscillation to one side is shown with a imaginary line), the direction of a magnetic flux flowing through the yoke 4 is changed to induce a voltage in a coil 6 wounded on this yoke 4.
However, according to this structure shown in FIG. 1, the pole parts of said flat armature 3 is slighly moved in its oscillation relative to the yoke 4 and since the size of the yoke 4 facing the armature as compared with that thereof in the direction of its magnetized thickness, is long, a lot of invalidated magnetic flux .phi..sub.B not crossing the coil 6 in addition to an effective magnetic flux .phi..sub.A crossing the coil 6 is produced and thus it is difficult to obtain good sensitivity. On the other hand, a cartridge shown in FIG. 2 is constructed as follows. A yoke 7 having E shaped section is disposed so that the center pole P.sub.1 thereof faces a flat armature 3 in static condition to leave a suitable space between it and the circumference of the armature 3. In inclination and oscillation of the flat armature 3 (A condition that it is tilted and biased by oscillation to one side is shown with an imaginary line), the circumference thereof approaches the front pole P.sub.2 of the yoke 7 or the rear pole P.sub.3 thereof, thereby causing the direction of a magnetic flux flowing through a part of the yoke on which a coil 6 is mounted to induce a voltage in the coil 6. In construction shown in FIG. 2, when the flat armature 3 is tilted and oscillated, one side of the pole in the circumference thereof approaches the center pole P.sub.1 in a magnetically unbalanced condition, but since variation of the distance between the other pole and the front pole P.sub.2 or the rear pole P.sub.3 is small, efficiency of generation of electricity is not high.
Further, in constructions shown in FIGS. 1 and 2, since the combination of the coil 6 and the yoke 4 or 7 on which it is mounted are adapted to detect oscillation in one direction, independently, respectively, in order to apply the combination to a general 2-channel stereo system, it is necessary to prepare two such combinations each having the same construction with each other for a left channel and a right channel. Moreover, owing to structure of the pole surface and the like, the yokes 4 and 7 made of magnetic material, for example, Permalloy must be bent in a .].-shape in a direction of its thickness and thus the magnetic characteristic thereof is in danger of deteriorating. Accordingly, such conventional construction is not desired in production of a yoke, easiness in assembling it and homogeniousness in the characteristic and product thereof.